The present invention relates to micromachining a sample, for example of semiconductor material, for observation of the sample by detection of electron or ion beam radiation transmitted through, or penetrating, the sample. Such observations are generally performed with a transmission electron microscope (TEM).
When a sample is observed by using TEM in the prior art, a region of the sample which is to be observed is processed to have a thickness in the submicron range, an electron beam is transmitted via a lens through the sample and a magnified image of the sample is formed on a photographic plate through a lens from the transmitted electron beam. In order for the electron beam to be transmitted through the sample, the sample must be made very thin. Production of a thin sample according to the prior art for the observation of the sample using the transmission electron microscope is very time consuming and labor intensive.
When a predetermined cross section of a semiconductor integrated circuit is observed by prior art techniques, a sample is mechanically machined to a thickness of more or less several tens of microns with a predetermined point which is to be observed located at a center region, and then the sample is etched by a wet or dry process from one or two sides adjacent the center region to a predetermined thickness for TEM observation.
But it is difficult in the prior art to obtain a thickness of 0.1-0.5 xcexcm of a sample for TEM observation, including the predetermined point, through etching by a wet or dry process. Especially it was difficult to keep the predetermined point for observation left on the sample as it is.
It is an object of the present invention to improve the preparation of a sample for observations of the above described type.
It is a particular object of the invention to prepare a sample for such observation while eliminating the shortcomings of prior art methods.
The above and other objects are achieved, according to the present invention, by a method for preparing a sample for observation, the sample having a surface, the method comprising:
contacting a first predetermined area of the sample surface with an organic compound vapor while irradiating the first predetermined area with an ion beam to decompose the organic compound into a layer having a mask function, the layer covering the first predetermined area; and
contacting a second predetermined area of the sample surface with an etching gas while irradiating the second predetermined area with an ion beam in order to remove material from the sample surface at the second predetermined area, wherein the second predetermined area includes at least part of the first predetermined area and the layer covering the first predetermined area prevents removal of material from the sample surface in the first predetermined area.
According to preferred embodiments of the invention:
the same ion beam is used in both contacting steps;
the ion beam is focussed;
each irradiating step comprises scanning the respective predetermined area with the focussed ion beam; and
each contacting step comprises spraying a respective gas at the respective predetermined area from a respective small diameter nozzle.
When the focussed ion beam repeatedly scans and irradiates a first predetermined area which is rectangular on a sample surface, while an organic compound vapor is sprayed through a small diameter spraying nozzle on that area, the organic compound absorbed on the sample surface at the first predetermined area is decomposed by the irradiation of the focussed ion beam. Such decomposed organic compound forms a film at the first predetermined area of the sample. The first predetermined area which is rectangular has a shape with a thickness sufficient to be observed by a TEM, with area being properly positioned for the TEM observation.
Next, simultaneously with the irradiation by the ion beam at the second predetermined area, which includes at least a part of the first predetermined area on which the above-mentioned film is formed, the film formed on the first predetermined area functions as a mask against etching and only sample surface areas not covered by the film and irradiated by the ion beam are etched due to the spraying of the etching gas, which magnifies the etching effect of the ion beam. Thus a thick sample cross section is obtained for TEM observation.